In situations where higher requirements in terms of operational, fire and contact safety have to be met when supplying electrical equipment (loads) with energy, the network type of an ungrounded power supply system is used. This network type is also known as an insulated network or IT system (French: isolé terre—IT). In this kind of power supply system, the active parts are separated from the ground potential, i.e. against ground. The advantage of these networks is that the IT power supply system can continue to operate in case of a first insulation fault, such as a fault to ground or a fault to frame, because the ideally infinitely large impedance value prevents a closed (fault) circuit from forming between the active conductors (outer conductors and potentially present neutral conductor) of the network and ground in this first fault case.
The inherent safety of the IT power supply system thus can ensure a continuous power supply of the equipment connected to the IT power supply system, i.e. of the loads fed by the IT power supply system, even if a first insulation fault occurs.
Hence, as a solution conserving energy and material, the IT power supply system in its configuration as a 400 V direct-voltage IT power supply system (DC-IT power supply system) including modern converter technology is also increasingly used in the field of telecommunication. There, the 400 VDC voltage is often led through kilometer-long lines toward the equipment. To save material costs, the protective conductor is omitted. The protective grounding of conductive, touchable parts of the equipment happens on site via an existing grounding of the building, similar to the independent grounding of equipment in the network type of the TT system (French: terre terre—TT).
A line break in such an extensive IT power supply system poses the risk of direct touching of an active conductor of the power supply system and may thus lead to a hazard to persons. Direct touching of one or both active conductors would not be safely detectable by a residual current device in the DC-IT power supply system because the fault circuit is not closed in an IT power supply system. While there should be no shock current in case of direct touching of only one active conductor in a DC-IT power supply system with a high level of insulation and little network leakage capacitances, direct touching of two active conductors is hazardous in a DC-IT power supply system, as well.
Even if an insulation monitoring device is used in a DC-IT power supply system to shut off the power supply in case of a first fault, i.e. upon triggering of a (main) alarm due to an insulation resistance falling short of an insulation resistance value, in most insulation monitoring devices, the automatic shut-off of the power supply upon direct touching of both active conductors will not be able to fulfill the protective measures required according to harmonized standard HD 60364-4-41:2007 “Low-voltage electrical installations—Part 4-41: Protection for safety—Protection against electric shock” for protection against electric shock.
If simultaneous touching of two active conductors in an IT power supply system is recognized as an inacceptable residual risk in the course of a risk assessment, measures have to be taken to reduce said residual risk.
Protective measures applied thus far, such as double or reinforced insulation or such as protection by safety extra low voltage (SELV) or by protective extra low voltage (PELV), are often contrary to the requirements to be met by a power supply system regarding supply reliability and in view of cost aspects.
For instance, the protective measures provided by SELV or PELV are often used in the field of telecommunication, too; however, they will no longer applicable because of their low voltages if 400 VDC power supply systems are to be employed for reasons of energy and material efficiency.
Thus, the object of the present invention is to provide a technical solution that minimizes the hazard in case of accidental touching of two active conductors in an ungrounded direct-voltage power supply system.